AD693 View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

AD693

Loop-Powered 4–20 mA Sensor Transmitter

Analog Devices 

AD693

increasing the application voltages by adding resistance between

Pins 14 and 3 will decrease the temperature span.

An external voltage divider may also be used in conjunction

with the circuit shown to produce any range of temperature

spans as well as providing zero output (4 mA) for a non 0

temperature input. For example, measuring VX with respect to a

voltage 2.385 times the excitation (rather than 2 times) will

result in zero input to the Signal Amplifier when the RTD is at

100°C (or 138.5 Ω).

As suggested in Table I, the temperature span may also be adjusted

by changing the voltage span of the Signal Amplifier. Changing the

gain from 2 to 4, for example, will halve the temperature span to

about 52°C on the 4-20mA output configuration. (See section

“Adjusting Input Span.”)

The configuration for a three-wire RTD shown in Figure 17 can

accommodate two-wire sensors by simply joining Pins 1 and 5

of the AD693.

INTERFACING LOAD CELLS AND METAL FOIL STRAIN

GAGES

The availability of the on-chip Voltage Reference, Auxiliary

Amplifier and 3 mA of excitation current make it easy to adapt

the AD693 to a variety of load cells and strain gages.

The circuit shown in Figure 18 illustrates a generalized approach in

which the full flexibility of the AD693 is required to interface to a

low resistance bridge. For a high impedance transducer the

bridge can be directly powered from the 6.2 V Reference.

Component values in this example have been selected to match

the popular standard of 2 mV/V sensitivity and 350 Ω bridge

resistance. Load cells are generally made for either tension and

compression, or compression only; use of the 12 mA zero tap

allows for operation in the tension and compression mode. An

optional zero adjustment is provided with values selected for

+2% FS adjustment range.

Because of the low resistance of most foil bridges, the excitation

voltage must be low so as not to exceed the available 4 mA zero

current. About 1 V is derived from the 6.2 V Reference and an

external voltage divider; the Aux-Amp is then used as a follower

to make a stiff drive for the bridge. Similar applications with

higher resistance sensors can use proportionally higher voltage.

Finally, to accommodate the 2 mV/V sensitivity of the bridge,

the full-scale span of the Signal Amplifier must be reduced.

Using the load cell in both tension and compression with 1 V of

excitation, therefore, dictates that the span be adjusted to 4 mV.

By substituting in the expression, RS1 = 400 Ω/[(30 mV/S) – 1],

the nominal resistance required to achieve this span is found to

be 61.54 Ω. Calculate the minimum resistance required by

subtracting 10% from 61.54 Ω to allow for the internal resistor

tolerance of the AD693, leaving 55.38 Ω (See “Adjusting Input

Span.”) The standard value of 54.9 Ω is used with a 20 Ω

potentiometer for full-scale adjustment.

If a load cell with a precalibrated sensitivity constant is to be

used, the resultant full-scale span applied to the Signal Amplifier is

found by multiplying that sensitivity by the excitation voltage.

(In Figure 18, the excitation voltage is actually (10 kΩ/62.3 kΩ)

(6.2 V) = 0.995 V).

THERMOCOUPLE MEASUREMENTS

The AD693 can be used with several types of thermocouple

inputs to provide a 4-20 mA current loop output corresponding

to a variety of measurement temperature ranges. Cold junction

compensation (CJC) can be implemented using an AD592 or

AD590 and a few external resistors as shown in Figure 19.

From Table II simply choose the type of thermocouple and the

appropriate average reference junction temperature to select

values for RCOMP and RZ. The CJC voltage is developed across

RCOMP as a result of the AD592 1 µA/K output and is added to

the thermocouple loop voltage. The 50 Ω potentiometer is

biased by RZ to provide the correct zero adjustment range

appropriate for the divider and also translates the Kelvin scale of

the AD592 to °Celsius. To calibrate the circuit, put the

thermocouple in an ice bath (or use a thermocouple simulator

set to 0) and adjust the potentiometer for a 4 mA loop current.

The span of the circuit in °C is determined by matching the

signal amplifier input voltage range to its temperature equivalent

Figure 18. Utilizing the Auxiliary Amplifier to Drive a Load Cell, 12 mA ± 8 mA Output

–10–

REV. A

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